Lunar supports adjusting brightness for most modern monitors.
If on the first try Lunar can't change the brightness of your monitor, try looking through this FAQ which lists most possible blockers of DDC and how to fix them if possible.
The most usual blockers are:
- The HDMI port of the Mac Mini (switch to a different port to allow DDC to work)
- Some monitor setting that blocks external brightness adjustments
- Dell has Uniformity Compensation which needs to be disabled
- LG has Picture Mode which needs to be set to Custom to allow DDC
- Prism has On-the-Fly Mode which needs to be set to Standard
- Samsung has Magic Bright and Input Signal Plus
- Most monitors have some kind of Color Preset that needs to be set to Standard or Custom
When DDC just doesn't work
If brightness doesn't change even after trying all of the above, Lunar still has a fallback where it can change the brightness by manipulating the Gamma settings in software. The monitor LED panel brightness stays the same, but the colours get darker as you lower the brightness in Lunar which mimics a brightness change.
I've written about how that works here if anyone's curious about it: The journey to controlling external monitors on M1 Macs
Lunar should fallback to Gamma automatically if DDC is detected unresponsive, but you can also force that fallback by disabling Hardware Controls in the Lunar monitor settings
Disclaimer: I'm the developer of Lunar